The present invention relates to material treatment processes, and more particularly to an enclosed solvent and aqueous decompression processing system that enhances the transfer of material to or from a liquid to a solid surface by producing bubbles at the solid surface and either detaching or collapsing these bubbles in a cyclical manner.
Cavitation is a well-accepted means of cleaning surfaces. An object having a solid surface to be cleaned is immersed in the fluid. Typically, ultrasonic sound waves are used to produce tiny collapsing bubbles at the solid surface. The energy of the ultrasonic waves is released into the fluid and the heat created by this energy evaporates small volumes of the fluid at the surface of the object, forming vapor bubbles. The vapor bubbles are cooled by the surrounding fluid and collapse, releasing their energy on implosion. The strength and aggressiveness of the imploding bubbles can be controlled by controlling the frequency and wavelength of the ultrasonic waves. Low frequency, long wavelength ultrasound produces smaller, less aggressive vapor bubbles that are usually used to cover more surface area and be less erosive to the material being cleaned.
Like ultrasound, decompression processing is the production of vapor bubbles at a solid surface, to produce an energy release at the solid surface. The process is accomplished by alternating vacuum and pressure to produce a pulsing action within a fluid. The release of pressure produces vapor bubbles at the solid surface, which are collapsed when pressure is re-applied. The level of vacuum, and/or pressure, the temperature, rate of introducing vacuum and/or pressure can control the rate of growth and size of the bubbles, and the total energy released.
It would be expected that the size of the bubbles produced with decompression processing can be much greater than that produced by ultrasound. The size and bubble production rate should be similar to that produced in a boiling liquid, which is directly proportional to the rate of heat addition. Since boiling vapor bubbles form at surface crevices and imperfections, it would be expected that decompression bubbles should be very selective by nucleating at particles on the surface thus enhancing particle detachment from the surface, i.e. removal of the particles from the surface (cleaning). If the bubbles are collapsed at the surface, the effect should be like ultrasound in that the imploding bubble would release a large amount of localized energy. On the other hand, if the vapor bubble is allowed to detach from the surface, the particle would be exposed to a reforming boundary layer, and this action should enhance transfer of material to a surface as required in surface coating processes. Unlike ultrasound bubbles which are micron-level in size, and generally smaller than the particles being removed, vapor bubbles formed by decompression would be larger and produce reforming viscous surface layers which can then have an effect on the particles.
These larger bubbles formed during decompression are more selective than ultrasound bubbles by forming at the particle sites, and it is expected that this could produce a targeted energy directed at the solid surface unlike ultrasound waves which release energy directly to the fluid. For sensitive surfaces, or surfaces with crevice particles, decompression indeed provides a more selective, less destructive means for particle removal. In addition, pressure effects of the decompression are omnidirectional throughout the fluid and thus are not shielded from any areas of the solid surfaces. In contrast, ultrasound waves are directional and thus certain surfaces of the solid may be shielded from their effects. Furthermore, since pressure equalizes in all directions, nucleating bubbles can be formed inside tubes just as easy as outside a tube.
The elimination of the fluid boundary layer during decompression may also enhance particle filtration processes especially when micron size particles are present. Generally, it becomes difficult to filter micron size particles from a liquid medium which contains particles which are smaller than 5 microns in diameter. This is because when the liquid is flowing through the filter matrix, the particles tend to follow the fluid streamlines more readily as the particle size is reduced. Micron size particles thus never reach the filter surface to be adsorbed and retained since the fluid velocity goes to zero at the solid filter surface. If the liquid near the filter surface is continuously removed by nucleating vapor bubbles, the micron size particles can now be carried to the surface by the fluid moving in behind the detaching bubble and the particle can now be adsorbed and retained at the surface.
The enhanced diffusion mechanism for particles described above can also be applied to liquid diffusion. For example if it is desired to deliver liquid to a surface for coating or other surface treatment, the evaporation of liquid from the surface can be rapid and the convective effect of the displacement fluid can be orders of magnitude greater than molecular diffusion. This method of diffusion can be more selective than conventional means. For example, in order to deliver an acid to a solid surface for etching, generally a highly concentrated acid solution may be required for performing the task. If a decompression process is used, evaporating bubbles will leave the acid behind creating a highly concentrated acid solution near the surface being treated. The constant flashing of fluid at the surface quickly decreases the pH of the solution used for etching while the surrounding fluid remains relatively high in pH thus not harming the treatment vessel or any other support piping or equipment.
In general, the present invention is directed to an enclosed solvent and aqueous decompression processing system including a chamber for holding an object to be processed. At least one vacuum pump applies a negative gauge pressure to the chamber to remove air and other non-condensable gases. Means are provided for introducing a solvent to the evacuated chamber to treat the object contained within. Treatment may be in the form of coating, etching, deposition, cleaning, stripping, plating, adhesion, dissolving, filtering or any other process in which material is removed or deposited on a solid surface by transfer from or to a liquid phase. A first system removes pressure from the chamber to produce vapor bubbles for processing. A second system increases pressure by ceasing to apply vacuum or adding non-condensable gases. The system includes recovery of the solvent from the chamber and object.
In another aspect of the invention, a method of treating an object in an enclosed solvent decompression processing system, including a solvent supply system in sealable communication with a cleaning chamber comprises the steps of:
(a) sealing the solvent supply system with respect to the chamber;
(b) opening the chamber to atmosphere and placing an object to be treated in the chamber;
(c) evacuating the chamber to remove air and other non-condensable gases;
(d) sealing the chamber with respect to atmosphere;
(e) opening the chamber with respect to the solvent supply system and introducing a solvent into the evacuated chamber;
(f) processing the object by cyclically alternating vacuum and pressure in the chamber;
(g) recovering the solvent introduced into the chamber;
(h) sealing the chamber with respect to the solvent supply system;
(i) introducing air into the chamber for sweeping further solvent on the object and within the chamber; and
(j) opening the chamber and removing the treated object.
The main objective of this invention is to enhance the transfer of material to or from a liquid to a solid surface by producing vapor bubbles at the surface and either detaching or collapsing these bubbles in a cyclical manner.
Another object of this invention is to provide an improved closed solvent decompression processing system and method which maintains solvent at a pure solvent vapor state, thus producing a thermodynamic state of a liquid in contact with its"" pure vapor. Under such conditions, when the liquid state properties vary only slightly, solvent is vaporized or condensed in a rapid manner. Varying the rates and magnitude of heat addition or removal or pressure increase or reduction in the chamber can control this system and change the characteristics of a process.
Another object of this invention is to provide an improved closed solvent decompression processing system and method which enables solvent recovery and limits hazardous emissions. The invention can employ a variety of solvents having boiling points as low as seventy degrees Fahrenheit and as high as 500 degrees Fahrenheit.
Other objects, features, and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings.